A technique which is common for use in making certain types of semiconductor devices, particularly semiconductor devices made of the Group III-V semiconductor materials and their alloys, such as light emitting devices and electron transfer devices is known as liquid phase epitaxy. Liquid phase epitaxy is a method for depositing an epitaxial layer of single crystalline semiconductor material on a substrate (generally a crystalline substrate) wherein a surface of the substrate is brought into contact with a solution of a semiconductor material dissolved in a molten metal solvent, the solution is cooled so that a portion of the semiconductor material in the solution precipitates and deposits on the substrate as an epitaxial layer, and the remainder of the solution is removed from the substrate. The solution may also contain a conductivity modifier which deposits with the semiconductor material to provide an epitaxial layer of a desired conductivity type. Two or more epitaxially layers can be deposited one on top of the other to form a semiconductor device of a desired construction including, for example, a semiconductor device having a p-n junction between adjacent epitaxial layers of opposite conductivity type.
Liquid phase epitaxial growth has been accomplished by various techniques including the method of slowly cooling the liquid phase which is in contact with a solid substrate as well as the method of employing a temperature gradient through the liquid phase. Further, various apparatus has been employed depending upon the method used. Where slow cooling has been used, the substrate has been contacted with the liquid phase by means of dipping, tipping, rotation or inversion of a substrate held in a container so as to contact the liquid and by means of an apparatus employing a slide wherein the liquid and substrate are contacted by sliding one over the other.
U.S. Pat. No. 3,565,702 to H. Nelson, issued Feb. 23, 1971 describes a method and apparatus for depositing one or more epitaxial layers by liquid phase epitaxy employing an apparatus which includes a furnace boat of a refractory material having a plurality of spaced wells in its top surface and a slide of a refractory material movable in a passage which extends across the bottoms of the wells. In the use of this apparatus, a semiconductor solution is provided in a well and the substrate is placed in a recess in the slide. The slide is then moved to bring the substrate into the bottom of the well so that the surface of the substrate is brought into contact with the solution. Upon cooling, epitaxial layers are deposited on the substrate and the slide is then moved to carry the substrate out of the well. A plurality of epitaxial layers may be deposited on the substrate by providing separate solutions and separate wells and carrying the substrate by way of the slide to each of the wells in succession. Other slide type apparatus have also being shown in the prior art all of which are included herein by reference. These prior art references are as follows: U.S. Pat. Nos. 3,767,481; 3,890,194; 3,933,538; 3,821,039; and 3,854,447.
In spite of the excellent crystalline characteristics of wafers grown in such slide apparatus by means of liquid phase epitaxy, certain problems still exist with use of such apparatus. In particular, epitaxially grown semiconductor films made by using the equilibrium cooling technique and prior art slide type apparatus results in the formation of excess edge growth. This excess edge growth has been explained as due to a greater heat loss from around the edges of the substrate well creating a temperature gradient along the substrate in the area of the well walls. When using the sliding boat type of apparatus, this edge growth not only reduces the usable area of a wafer due to the nonuniformity of the thickness of the epitaxial layer across the wafer, but it also interferes with the sliding mechanism of the apparatus. Further, as the substrate is moved from one melt to the next, excess edge growth can break off and be dragged over the grown layer producing scratches. To minimize this effect, a clearance between the substrate and graphite may be employed. However, such a clearance, while eliminating or reducing a problem of scratches, often results in a measurable amount of material being carried between successive melt. I have now discovered a method of altering the thermal properties of a melt well so as to substantially eliminate or reduce the excess edge growth.